In recent years, the surface treatment technique using a fluororesin has been extensively utilized. Fluororesins exhibit various properties such as non-tackiness, heat resistance, oxidation resistance, weather resistance, chemical resistance, sliding properties, water/oil repellency, and electrical properties because of their characteristic structures. Utilization of various functionalities of a coating film formed using a functional coating liquid such as a solution, slurry, or paste (hereinafter, also together written as “slurry”) containing a fluororesin has been studied in order to exhibit these properties in various fields such as paints, inks, coating agents, covering materials, building materials, kitchenware, household utensils, OA apparatuses, automobiles, medical treatment, electronic materials, semiconductors, liquid crystals, home electrical appliances, and electrical storage apparatuses.
Polyvinylidene fluoride (hereinafter, also written as “PVDF”) is generally used in consideration of its coating applicability and the like as a fluororesin for use in coating liquids and coating agents. One of the properties that a coating film should have is an excellent adhesiveness to a base material. This is because it is when the adhesiveness to the base material is excellent that the functionalities of the coating film are fully exhibited. However, PVDF shows non-tackiness to general base materials, and therefore use of a coating liquid that merely contains PVDF results in an insufficient adhesiveness of the coating film to be formed to abase material. Accordingly, there are various proposals to improve adhesiveness of a coating film containing PVDF to a base material.
For example, in the field of paints for exterior building materials, there is a proposal in which a coating film is formed blending an acrylic resin with PVDF, and then a film or another coating film is further laminated on the coating film (Patent Literatures 1 and 2). Moreover, it is proposed to use a modified fluororesin obtained by introducing a functional group or a modified substance by which an effect of improving adhesiveness is expected into a basic repeating unit (Patent Literature 3 and 4).
On the other hand, in the field of batteries, PVDF is widely used as a binder for binding active materials in a lithium ion battery, and a solution of a binder, the solution obtained by dissolving PVDF in an NMP solution, is generally used as a dispersion medium. The amorphous moiety of PVDF suitably functions as a matrix for a polar solvent or an electrolyte in an electrolytic solution, and PVDF does not completely obstruct diffusion of lithium ions (Non Patent Literature 1). However, when an electrode is prepared using a solution of a binder, the solution obtained by dissolving PVDF in an NMP solution, the binding force of active materials may be insufficient. Therefore, various measures to improve the binding force of active materials have been studied.
For example, there is proposed an electrode obtained by binding primary particles of a positive electrode active material with a water-soluble polymeric binder and binding secondary particles, or a secondary particle and a collector with a fluororesin-based binder or a rubber-based binder (Patent Literature 5). Moreover, it is proposed to use a modified PVDF obtained by subjecting PVDF to partial dehydrofluorination with an alkali and thereafter further subjecting the treated PVDF to oxidation treatment with an oxidizing agent (Patent Literatures 6 and 7).